講演情報
[PPS01-P09]Morphological and Reflectance Variations of Jupiter’s Great Red Spot Using Visible Imaging Data
*高木 聖子1、河野 晃大1、木村 淳2 (1.北海道大学、2.大阪大学)
キーワード：
木星、大赤斑
The Great Red Spot (GRS), a defining feature of Jupiter, is one of the longest-lived atmospheric vortices in the Solar System. Temporal variations in its size and shape remain central topics in planetary atmospheric dynamics, and recent studies have reported a long-term contraction trend. Historical records date back to the mid-1800s, when its position and size were measured using small telescopes [Peek 1958; Rogers 1995]. Early observations primarily recorded its longitudinal extent and transit times. With the development of imaging techniques, quantitative assessments of its shape became possible. In the early 1900s, the longitudinal extent of the GRS was estimated to be approximately 40,000 km, and subsequent observations have shown a gradual decrease in this value [Simon et al., 2018].
Starting with close-range imaging by the Voyager and Pioneer missions, continuous monitoring by the Hubble Space Telescope and high-spatial-resolution observations by spacecraft such as Galileo, Cassini, New Horizons, and Juno have provided extensive data on the shape, structure, and temporal variability of the GRS [Simon-Miller et al., 2002; Asay-Davis et al., 2009; Shetty and Marcus, 2010; Simon et al., 2014; Simon et al., 2018]. Simon et al. (2018) reported that between 1978 and 2017, the longitudinal extent decreased at an approximately linear rate of -0.194°/year, while its latitudinal extent decreased at approximately -0.048°/year. The more rapid contraction along the major axis indicates that the GRS is not only shrinking in size but also changing its aspect ratio, evolving from a more elongated ellipse toward a more circular shape. Numerical simulations based on vortex dynamics suggest the GRS may gain energy through interactions with and assimilation of surrounding small-scale anticyclones and storms [Asay-Davis et al., 2009; Shetty and Marcus, 2010]. In addition, the GRS is located between strong east-west jet streams, and dynamical interactions with adjacent zonal flows may also influence its morphology [Marcus 1993; Simon et al., 2014]. Recent observations indicate that the optical properties have also changed over time. Since 2014, reflectance at short wavelengths (< 650 nm) has decreased, while brightness at 890 nm has increased [Simon et al., 2018], suggesting variations in high-altitude clouds and haze. Consequently, the temporal variation of the GRS should be understood as a phenomenon resulting from the combined effects of changes in the dynamical scale of vortex and variations in its optical structure, including clouds and haze.
However, it remains unclear whether the previously reported contraction rates have persisted in recent years or whether the reflectance variations or inflection points have emerged in recent years. It is also uncertain whether the reflectance variations follow a monotonic trend or are correlated with morphological changes. A consistent comparison of shape evolution and optical variability is essential for understanding the long-term stability and internal structural changes of the GRS. While spacecraft and space telescopes provide high spatial resolution, their observations are temporally limited. In contrast, ground-based telescopes allow continuous monitoring despite their lower spatial resolution. The Pirka Telescope at Hokkaido University, equipped with 1.6 m primary mirror, is among the largest ground-based telescopes dedicated to Solar System observations and maintains an archive of visible images of Jupiter spanning approximately the past decade. In this study, we analyze these data in unified framework to evaluate temporal variations in longitudinal and latitudinal extent, morphology, and reflectance. Our objective is to examine whether the previously reported linear contraction trend continues and to quantify any recent changes in the contraction rate or the presence of inflection points.
Starting with close-range imaging by the Voyager and Pioneer missions, continuous monitoring by the Hubble Space Telescope and high-spatial-resolution observations by spacecraft such as Galileo, Cassini, New Horizons, and Juno have provided extensive data on the shape, structure, and temporal variability of the GRS [Simon-Miller et al., 2002; Asay-Davis et al., 2009; Shetty and Marcus, 2010; Simon et al., 2014; Simon et al., 2018]. Simon et al. (2018) reported that between 1978 and 2017, the longitudinal extent decreased at an approximately linear rate of -0.194°/year, while its latitudinal extent decreased at approximately -0.048°/year. The more rapid contraction along the major axis indicates that the GRS is not only shrinking in size but also changing its aspect ratio, evolving from a more elongated ellipse toward a more circular shape. Numerical simulations based on vortex dynamics suggest the GRS may gain energy through interactions with and assimilation of surrounding small-scale anticyclones and storms [Asay-Davis et al., 2009; Shetty and Marcus, 2010]. In addition, the GRS is located between strong east-west jet streams, and dynamical interactions with adjacent zonal flows may also influence its morphology [Marcus 1993; Simon et al., 2014]. Recent observations indicate that the optical properties have also changed over time. Since 2014, reflectance at short wavelengths (< 650 nm) has decreased, while brightness at 890 nm has increased [Simon et al., 2018], suggesting variations in high-altitude clouds and haze. Consequently, the temporal variation of the GRS should be understood as a phenomenon resulting from the combined effects of changes in the dynamical scale of vortex and variations in its optical structure, including clouds and haze.
However, it remains unclear whether the previously reported contraction rates have persisted in recent years or whether the reflectance variations or inflection points have emerged in recent years. It is also uncertain whether the reflectance variations follow a monotonic trend or are correlated with morphological changes. A consistent comparison of shape evolution and optical variability is essential for understanding the long-term stability and internal structural changes of the GRS. While spacecraft and space telescopes provide high spatial resolution, their observations are temporally limited. In contrast, ground-based telescopes allow continuous monitoring despite their lower spatial resolution. The Pirka Telescope at Hokkaido University, equipped with 1.6 m primary mirror, is among the largest ground-based telescopes dedicated to Solar System observations and maintains an archive of visible images of Jupiter spanning approximately the past decade. In this study, we analyze these data in unified framework to evaluate temporal variations in longitudinal and latitudinal extent, morphology, and reflectance. Our objective is to examine whether the previously reported linear contraction trend continues and to quantify any recent changes in the contraction rate or the presence of inflection points.